A voltage regulator is a power quality device that provides a stable output voltage despite fluctuations in an input voltage. Voltage regulators are typically located at distribution substations and on single power line feeders. For example, if an input voltage fluctuates between 110 VAC and 130 VAC, the voltage regulator maintains the output voltage at a constant 120 VAC. The voltage regulator operates by comparing the actual output voltage (which is either measured directly or calculated as described below) to a fixed reference voltage set point (a user-defined setting). The reference voltage setpoint is typically stored within a regulator control panel, which controls operation of the regulator. The regulator control panel determines the difference between the actual output voltage and the reference voltage set point and uses this difference to control a regulating element. Generally, a voltage regulator control panel comprises electronic components with input and output signals and attendant software programs to control regulator operation.
One example of a regulating element is a tap changer that establishes the winding ratio between a primary and a secondary transformer winding. A motor controls a position of the tap changer; operating the tap changer changes the winding ratio and thus the transformer secondary winding output voltage. The voltage regulator control panel controls the position of the tap changer to reduce the difference between the regulator output voltage and the set point to a value within a user-defined bandwidth, typically between about 1 and 6 volts.
Voltage regulators are sized for operation on distribution systems with nominal voltages between about 2400 VAC and 34,500 VAC. By controlling the tap position of the tap changer the distribution voltage can be varied by about ±10%.
One voltage regulator used in the electrical power industry increases or decreases its output voltage by up to 10% of its input voltage in ⅝% steps. Adjusting the transformer tap changer to one of thirty-two tap changer positions, each position providing a ⅝% adjustment, produces the total adjustment range of about ±10%. Typically, a tap changer comprises eight different available connections (i.e., eight different voltages) to a series winding. A preventive autotransformer (also known as a bridging reactor) permits operation in either a bridging or a non bridging mode. In the bridging mode, two movable tap changer contacts are connected to two different voltages (e.g. to two different stationary contacts) on a series winding, effectively providing a voltage that is halfway between the voltages at the two contacts. Operation in the bridging mode allows the tap changer to operate without causing voltage interruptions in the output voltage. Finally, a reversing switch (having positions K and M as illustrated in the figures herein) changes the winding polarity and further doubles the available tap positions. Thus a total of thirty-two tap positions are available, sixteen positions raise the regulator output voltage and sixteen positions lower the voltage.
During periods of peak demand the voltage on an electrical power distribution system can drop, especially if the system uses relatively small-diameter conductors. As additional current is demanded, the relatively high resistance of the small-diameter conductors increases the voltage drop through the conductors, thus lowering the system voltage. The voltage regulator can compensate this voltage drop by monitoring the distribution system voltage and knowing the fixed reference voltage set point (i.e., the desired output voltage), operate the tap changer to increase the output voltage supplied to the distribution system.
Fluctuations (up or down) in a source or input voltage to the voltage regulator can also be compensated by operating the tap changer to supply an output voltage at the reference voltage. Thus the voltage regulator maintains a stable output (load) voltage despite changes in the load demand (current) or changes in the source voltage.
The use of voltage regulators is also mandated by the increasing number of interconnects between power system regions and the use of different system voltages by power system operators.
With the increasing complexity of power system interconnects and the use of automatic power restoration devices, current (power) may flow in a forward or a reverse direction through an electrical power distribution system. Obviously, reverse current (power) flow also causes power flow in the reverse direction through the voltage regulator. Thus the voltage regulator must be capable of operating under both forward and reverse power flow conditions.
There are two common types of voltage regulators, referred to as Type A and Type B regulators. Each regulator type may also include a potential transformer. With both forward and reverse power flow controlled by the Type A and B regulators (with and without a potential transformer) there are therefore eight different possible scenarios.
The references to the voltage regulator “source bushing side” (the S and SL bushings or terminals) and the voltage regulator “load bushing side” (the L and SL bushings or terminals) in the discussion below of the Type A and B regulators refer to the regulator bushings or terminals and do not refer to the direction of current (power) flow. For forward power flow, power is supplied to the S and SL terminals and the output side comprises the L and SL terminals. For reverse power flow, power is supplied to the L and SL terminals and the load is connected across the S and SL terminals. Thus the “source side” and the “load side” are reversed when the power flow direction reverses.
An ANSI Type A voltage regulator 10 illustrated in FIG. 1 includes S and SL bushings or terminals with an exciting winding 12 disposed between the S and SL bushings. A series winding 14 forms an autotransformer with the exciting winding 12; the voltage taps are disposed on the series winding 14. The voltage regulator 10 further comprises L and SL bushings or terminals. Forward power flow direction in FIG. 1 is indicated by an arrowhead 15, i.e., forward power flow from the S/SL terminals to the L/SL terminals.
An output voltage of the regulator 10 at the terminals L/SL (for forward power flow) is controlled by a position of a tap changer 18 on the series winding 14 through bridging and non-bridging connections to the stationary contacts 0-8 connected to the series winding 14. The tap changer 18 comprises stationary contacts 19 and moving contacts 22. The moving contacts 22 are connected to a preventive autotransformer 24 (which, among other features, allows tap changer operation without power interruption). The autotransformer 24, which supplies the output voltage from the regulator 10, is connected to the L terminal or bushing through a conductor 25.
A switch 28 connected to the S terminal or bushing comprises a switch wiper 28A connected to the exciting winding 12. The switch 28 can be closed through terminal M or terminal K of the series winding 14 to change the polarity of the voltage through the winding 14. This polarity selection, together with the available tap positions, provides an output voltage adjustment through a range of about −10% to about +10% of the input voltage to the voltage regulator.
A tertiary (utility) winding 30, responsive to magnetic flux generated by the exciting winding 12, supplies power to a voltage regulator controller that in turn controls a tap changer motor (neither illustrated in FIG. 1) to operate the tap changer 18 to a desired tap position. The tertiary winding 30 also supplies power to other components of the voltage regulator 10, including the control panel.
The voltage regulator 10 further comprises a current transformer (CT) 34 and a potential transformer (PT) 36, both on the L/SL bushing side of the voltage regulator 10. The CT 34 produces a current that is a fraction of the load current flowing from the autotransformer 24 to the load through the L terminal. The CT current is used for instrumentation purposes.
The PT 36 generates a voltage that is a fraction of the load voltage across the L/SL terminals. The PT 36 comprises a primary winding 36A connected across the L and SL terminals and a secondary winding 36B that generates a voltage responsive to the primary winding voltage. The ratio of the primary to the secondary voltage is determined by the turns ratio between the primary and secondary windings. The PT has a conventional or standard turns ratio between the primary and secondary winding, such as a ratio of 60:1.
In operation during forward power flow in the direction indicated by the arrowhead 15, the regulator measures the voltage (the load-side voltage) across the PT secondary winding 36B and determines a difference between the PT secondary winding voltage and a reference set point or desired voltage. The regulator controller then controls the tap changer motor to operate the tap changer 18 to reduce this difference to about zero. By controlling the tap changer 18 to control the PT secondary winding voltage to a desired value, the output of the regulator across the L and SL terminals (the load side in this configuration) is thereby controlled to a desired value.
Consider operation of the regulator 10 during reverse power flow as indicated by an arrowhead 39. The PT 36 is positioned across the L and SL terminals, but for reverse power flow the L and SL bushings are on the source side of the regulator. The output or load voltage is between the S/SL terminals. In this scenario, the voltage across the tertiary winding 30 (now on the load side) is measured and the tap changer controller controls the tap changer motor to operate the tap changer and attain the desired output or load voltage across the tertiary winding or the S/SL terminals.
It is known that a typical tertiary winding 30 has a non-standard turns ratio (for example, not a 60:1 or a 63.5:1 ratio, nor a ratio that generates 120 VAC, 115 VAC or 125 VAC in the tertiary winding) that must be corrected prior to using the tertiary winding voltage to calculate (and then control) the regulator output voltage as described above. This correction is required to normalize the PT output voltage to a conventional or standard turns ratio. The normalized or corrected tertiary winding voltage can be used to control the tap changer during forward power flow and during reverse power flow.
In certain prior art voltage regulators, the tertiary winding non-standard turns ratio is corrected or converted to a standard turns ratio by connecting the tertiary winding to a primary winding of a dry-type ratio correction transformer. The secondary winding of the ratio correction transformer is connected to the control panel. This ratio correction transformer can typically change the tertiary winding voltage by between about +/−1 volt and +/−20 volts. This correction ensures that the voltage input to the control panel from the secondary winding of the ratio correction transformer (for both the forward and reverse power flow cases) represents a voltage supplied from a transformer with a standard turns ratio.
To eliminate costs associated with the PT 36 of FIG. 1, a PT is not present in another prior art voltage regulator, i.e., an ANSI Type A regulator without a PT as illustrated in FIG. 2 and designated by reference character 50. For the forward direction of power flow indicated by the arrowhead 15, the regulator 50 measures the source-side voltage across the tertiary winding 30 (correcting the voltage to a conventional turns ratio as described above using a dry-type transformer). The control panel calculates the load-side voltage across the L and SL bushings from the measured and corrected tertiary winding voltage and from a position of the tap changer 18, as that position is tracked, updated and stored in a control panel memory. The controller controls the tap changer motor to move the tap changer 18 based on the calculated output voltage and a desired reference voltage. Commonly-owned U.S. Pat. No. 7,023,193, which is incorporated herein by reference, discloses and claims such a voltage regulator.
In a case of reverse power flow through the regulator 50, as indicated by the arrowhead 39, the load-side voltage is measured across the tertiary winding (correcting the voltage as described above). This load-side voltage is used by the voltage regulator 50 to control the tap changer 18 to produce the desired output voltage from the voltage regulator 50.
In the various scenarios described above, once the load-side voltage is known (either measured directly or calculated) the tap changer is controlled to supply the desired regulated load-side voltage. As described above, the Type A regulator, either with or without a PT, can accommodate both forward and reverse power flows.
Certain installations use an ANSI Type B regulator (also referred to as an “inverted” regulator) such as illustrated in FIGS. 3 and 4. There are no operational or functional differences between Type A and B regulators; they can be used interchangeably. A Type B regulator comprises many of the same components as a Type A regulator.
Forward power flow through a Type B regulator 70 without a PT is illustrated in FIG. 3 with the forward power flow direction indicated by the arrowhead 15. The voltage across the tertiary winding 30 (connected across the L and SL bushings, which for forward power flow represents the load side) is measured and corrected to a standard turns ratio as described above. The corrected load-side voltage is input to the voltage regulator controller and compared with the desired reference or set point voltage. The difference between these two values controls the tap changer motor, operating the tap changer to change the load-side voltage to the desired value.
For reverse power flow, as indicated by the arrowhead 39 in FIG. 3, the regulator 70 measures the source-side voltage across the tertiary winding 30 (correcting the voltage as described above). The control panel calculates the load-side voltage across the S and SL bushings from the measured and corrected tertiary winding voltage and from a position of the tap changer 18 as stored in memory. Responsive to the calculated load-side voltage, the control panel controls the tap changer 18 to produce the desired output voltage.
A Type B regulator 80 in FIG. 4 includes the PT 36 between the S and SL terminals. For forward power flow, as indicated by the arrowhead 15, the voltage across the tertiary winding 30 (the load side) is measured and corrected as described above. This voltage is used to control the tap changer 18 to produce the desired load-side voltage.
For reverse power flow as indicated by the arrowhead 39 in FIG. 4, the voltage across the PT 36 (the load side) is measured and the measured voltage input to the regulator controller for operating the tap changer 18 to provide the desired output voltage.
The majority of voltage regulators include a tertiary winding to generate voltages that power components of the voltage regulator. Many voltage regulators do not include a PT. In fact, a PT is not required for proper operation of the voltage regulator. But in those installations where a voltage regulator does not include a tertiary winding, a PT is installed in lieu of the tertiary winding; this PT is typically installed on the load bushing side of the voltage regulator. However, there is no application or operational differences between a voltage regulator without a tertiary winding and the regulator embodiments described above.
Some utilities require voltage regulators with external test terminals for use by a technician to attach voltmeter probes and read a voltage on a voltmeter display. However, the ability to measure and display these voltages is not necessary for proper operation of the voltage regulator.
There are several industry-standard common turns ratios for a potential transformer, such as 7200:120 (or 60:1). Thus for a voltage regulator including a PT, a service technician measures the PT secondary voltage with a hand held voltmeter or similar instrument and expects to read a voltage that is a 60:1 fraction of the line voltage.
For those voltage regulators that include a tertiary winding, the tertiary winding voltage can also be supplied to external test terminals for measuring by a service technician. Although tertiary windings have an accurate turns ratio, they typically do not have a standard or conventional turns ratios, such as 7200:120. Instead, a tertiary winding may exhibit a winding ratio of 7200:117 due to limitations imposed during the manufacturing process. Although these non-standard voltages could be made available at external test terminals, these voltages are not easily interpreted by a service technician accustomed to working with transformers that produce “standard” voltages generated by transformers with a “standard” winding ratio, such as potential transformers. Thus many utilities require scaling of the tertiary winding voltage to reflect a common turns ratio for measuring at the external test terminals.
To satisfy these requirements, certain voltage regulators have the ratio correction transformer mounted outside the voltage regulator for access by a service technician. For example, the ratio correction transformer comprises connections to provide a ratio of 117:120 to overcome the effect of the non-standard turns ratio of the tertiary winding. Thus the output voltage from the ratio correction transformer provides a familiar voltage that reflects a conventional transformer turns ratio, for example 60:1 for the tertiary winding operating in combination with the ratio correction transformer. These ratio-corrected voltages are supplied to the measurement input of the control panel and are subsequently made available at the external test terminals to meet the utility's requirements for an external test terminal that provides a voltage based on a conventional transformer turns ratio.
As an alternative to using the ratio correction transformer, some prior art voltage regulator control panels use a numerical correction factor to adjust a first digital value, the first digital value representing a voltage across the tertiary winding, to a second digital value that represents an output voltage from a transformer with a standard turns ratio.
Specifically, this numerical correction factor, which is fixed according to a fixed predetermined regulator setting, adjusts the digital representation of the RMS analog voltage from the tertiary winding as that value is stored in the control panel memory. For example, as described above, a voltage regulator may use a tertiary winding with a transformer ratio of 7200:117 to determine the voltage on the load side of the voltage regulator. The control panel numerically adjusts the digital representation of the analog RMS value stored in memory to generate a new digital value that reflects the desired transformer turns ratio of 7200:120.
However, the digital value is not supplied to external test terminals for use by service technicians in measuring a voltage regulator source voltage or load voltage. In fact, the digital value has no meaning to service technicians as they are familiar only with the measurement of AC values at the external test terminals.
This alternative technique for correcting the non-standard turns ratio of the tertiary winding poses a disadvantage by eliminating the ratio correction transformer. Without the ratio correction transformer there is no analog voltage to supply to the regulator external test terminals for measuring by a lineman or service technician. It is customary in the utility industry for the service technician to measure the regulator output voltage using a handheld voltmeter. In this case, the voltage measurement is ratio corrected at the digital level and therefore the voltage to be measured at the test terminals is not ratio corrected. If the service technician performing the measurement does not consider this, he will misinterpret the voltage measured at the test terminals and can inadvertently manually raise or lower the voltage beyond prescribed limits.
Many utilities require external test terminals where a regulated voltage based on a standard turns ratio is available for both forward and reverse power flows. For a voltage regulator providing bidirectional regulation, this feature requires both a PT and a combination of a tertiary winding and a ratio correction transformer. One of these transformers determines the load-side voltage for forward power flow and the other determines the load-side voltage for reverse power flow.
For a Type A regulator providing only forward power flow regulation, customers typically require a PT (which for a Type A regulator is disposed on the load bushing side), in addition to the tertiary winding on the source bushing side for powering the regulator. See the prior art voltage regulator illustrated in FIG. 1. For a Type B regulator a ratio correction transformer is required for use with the tertiary winding disposed on the load bushing side. See FIG. 4.
Thus as can been seen, in the prior art additional hardware components are required to produce “standard” voltages and make them externally available to meet utility requirements and preferences.
The inventor has recognized that avoiding use of the ratio correction transformer substantially reduces the voltage regulator cost. If the PT can also be eliminated, the cost is further reduced.